Ultraviolet light emitting diodes emit light of wavelength ranging from 100 nm to 400 nm and are classified into UVA, UVB, and UVC in the industry depending on the nature of the generated light.
Commercialized ultraviolet light emitting diodes are based on AlGaN material and is characterized by having emission wavelength determined by the fraction of Al. That is, higher fraction of Al in quantum wells results in shorter wavelength light. Also, while the manufacturing process is similar to the conventional blue light emitting diodes, there are specific differences for formation of p-type semiconductor layers.
That is, in order to form a p-type semiconductor layer in an ultraviolet light emitting diode, Mg is used as a dopant in an AlGaN compound semiconductor layer. However, the difference between the valence band energy in AlGaN and the electronic energy levels of Mg is 0.35 eV. This energy difference is very large and contributes to the difficulty activating the dopants in a p-type semiconductor layer to form holes.
For example, in the case of a p-type semiconductor layer used in a blue light emitting diode, the difference between the valence band energy in GaN and the electronic energy levels of Mg is 0.15 eV to 0.17 eV. In order to form holes in GaN by activating the dopants, electrons in GaN need to move to the Mg electronic levels, and, for this to happen, energy larger than 0.15 eV to 0.17 eV needs to be supplied. In order to achieve this, in a blue light emitting diode, a p-type semiconductor layer is formed using an MOCVD process, and a subsequent thermal processing at about 700° C. or higher is accompanied. However, even in the case of performing a thermal processing, the extent of activation is known to remain at about 10%.
On the other hand, in an ultraviolet light emitting diode utilizing AlGaN as a the base material, even in the case of using Mg dopant in order to form a p-type semiconductor layer, there is significant limitation to forming holes. That is, because the energy level difference between AlGaN and Mg is relatively very high, dopant activation level is low. It is known in the industry that the dopant activation level is approximately 1% to 3%. Accordingly, even in the case of forming a p-type semiconductor layer using AlGaN, the p-type semiconductor layer has high resistivity due to low hole density, and the amount of holes supplied to a quantum well structure is small such that light emitting action with enough brightness cannot be achieved.
To address the aforementioned problem, a p-type GaN layer is additionally introduced on top of the p-type AlGaN. That is, through the introduction of a p-type GaN layer, a hole density necessary for light emitting action can be achieved. However, the above described structure has a serious problem during light emitting action. This is because ultraviolet light generated in a quantum well structure is absorbed by the p-type GaN layer due to the bandgap energy of the GaN layer.
Therefore, ultraviolet light generated in a quantum well structure is significantly prevented from escaping outside due to mostly being absorbed.